In both cases, it is the center plane of the feature of size. In the first example, it is the center plane of the 7-inch feature. In the second, it is the center plane between the two sides. Same thing, just that the second example does not include a dimension. Note that in both cases it is the center plane between faces, not edges.

A data simulator for a center plane can be a special piece of equipment that is like a vise or clamp, but, in cases I've seen, the planes for both faces are found on a CMM and the center plane determined mathematically.

Note that, in the first example, the center lines are shown extending between views. That is non-standard for ASME Y14. The centerlines should not extend beyond a view.

Just a quick question. Are the B and C datums actually aligned with the 7.00 and 5.00 dimensions extension arrows? Because if so then they are centered on the feature and if not they just show other locating planes for measurement. The view looks a little squished, IMHO, for this viewing purpose.